The need for weight reduction in mechanical drives has in the past lead to an increased use of planetary gear units.
In a number of applications, more and more integration and lightweight designs are being introduced. This leads to a higher importance of deflections and local deformations. This applies particularly in the case of a gear unit for use in a wind turbine and for which application it has been proposed optionally to utilise a ring gear to act as a support directly for an inner or outer bearing ring.
This invention addresses the consequence on bearing performance of local deformation of the ring gear of a planetary gear stage by the passage of the planets in the relative motion of the gear system.
As shown diagrammatically in FIG. 1, the resultant gear force (Fn) acting at the gear contact between a planet gear 10 and ring gear 11 includes a radial (Fr) and a tangential (Ft) component. Also axial forces may occur for instance when helical gears 10 are used.
Although the present invention seeks to consider all components of the occurring forces, it is to be understood that for instance the radial force can lead to significant local elastic deformations of the ring gear. When the gear unit is in operation this deformation will run through the ring gear at a speed which is synchronous with the moving planets i.e. with the planet carrier 12.
The magnitude of this deformation will depend on the forces and the surrounding structural stiffnesses.
In typical state-of-the-art planetary gear units, the planet carrier 20 (see FIG. 2) is mounted in bearings 21 which center in the ring gear 22 via an intermediate flange or housing 23.
This means that any local deformations of the ring gear due to the passage of the planets will be distributed more evenly by this intermediate flange or housing. Thus when load is applied to the bearings, for instance external forces from the rotors of a wind turbine, they will be operating with loads that are relatively well distributed over the different rollers of the bearings.
FIGS. 3a to d show examples of a more integrated and lightweight design, where the application, such as a wind turbine rotor, can be directly, or via a flange, connected to the planet carrier (FIG. 3a, 3b, 3c) or to the ring gear (FIG. 3d). Also external forces which may come from the connected application will have to be supported by the construction.
Because of the integration, the structural stiffnesses of the system change and forces acting in the gear contact between planets and ring gear can now lead to significant local deflections of the bearing(s)                in FIGS. 3a, 3c and 3d, the outer ring 30 of the bearing(s) will deform locally at the passage of the planets 31.        in FIG. 3b, a variant of FIG. 3a, the inner ring 32 of the bearing will deform locally at the passage of the planets 34.        
As can be seen from the examples in FIG. 3, it is always the bearing ring connected to or integrated with the ring gear 35 which suffers from the above type of local deformation. By definition herein, we call this bearing ring A, whereas the other bearing ring which rotates synchronously (including standing still) with the planet carrier is called bearing ring B.